Efficacy of levothyroxine monotherapy in achieving clinical euthyroidism and its impact on weight loss in women with hypothyroidism and obesity.

In light of research indicating that normal TSH levels do not necessarily reflect clinical euthyroidism in hypothyroid patients, and given the patient dissatisfaction with levothyroxine (LT4) monotherapy, we evaluate the response to weight loss treatment in patients with obesity and primary hypothyroidism who are euthyroid under LT4 therapy, with a focus on weight and metabolic parameters. The retrospective study included 138 subjects with obesity (BMI ≥ 30 kg/m), 69 women with hypothyroidism on levothyroxine, and 69 age- and BMI-matched women without thyroid disease. Secondary causes of obesity, medications that may affect thyroid functions and metabolic parameters, chronic and oncological disease, pregnancy, TSH outside the reference range, follow-up periods of less than one year, and bariatric surgery were the exclusion criteria. Patients' characteristics and metabolic responses to weight reduction treatment consisting of calorie restriction, moderate exercise, and metformin if needed were evaluated. TSH and fT4 levels were higher, and fT3 and T3/T4 ratios were lower all within the reference ranges in hypothyroid women. They were less insulin resistant. Both patient' groups experienced a significant decrease in body weight, BMI, and atherogenic index of plasma (AIP) during the follow-up (p = 0.001). Insulin resistance was not changed. The groups' body weight, BMI, HOMA-IR, and AIP changes were similar at the end of the study (p = 0.876, p = 0.850, p = 0.555, p = 0.293). The results suggest that achieving euthyroid status via levothyroxine monotherapy in hypothyroid women leads to weight loss responses comparable to those in women with normal thyroid. This supports the effectiveness of current hypothyroidism treatment strategies, emphasizing TSH level normalization, in achieving clinical euthyroidism concerning weight loss outcomes.